1. Field of Invention
The present invention relates to a pressure control device. More particularly, the present invention relates to a pressure control device for controlling the pressure within the ink reservoir of an ink-jet pen.
2. Description of Related Art
Conventional ink-jet printing generally relies on the controlled delivery of ink droplets from an ink-jet pen ink reservoir to a print medium. Among the printing methods for delivering ink drops from the ink reservoir to the print head, drop-on-demand printing is known as the commonly used method. Drop-on-demand method typically uses thermal bubble or piezoelectric pressure wave mechanisms. A thermal bubble type print head includes a thin film resistor that is heated to cause sudden vaporization of a small portion of ink. The vapid expansion of the ink vapor forces a small drop of ink through a print head nozzle.
Although drop-on-demand printing is ideal for sending ink drops from an ink reservoir to the print head, some mechanism must be included to prevent ink leaking out from the print head when the print head is inactive. Such a mechanism usually can build a slight back pressure at the print head to prevent ink leakage from the pen whenever the print head is inactive. Herein, the term "back pressure" represents the partial vacuum within the ink reservoir. Back pressure is defined in the positive sense so that an increase in back pressure means the degree of partial vacuum has increased.
When back pressure is established at all times inside the reservoir, ink is prevented from permeating through the print head. However, the back pressure can not be so high that the print head is unable to overcome the back pressure to eject ink drops. Furthermore, as ambient air pressure decreases, a corresponding greater amount of back pressure is needed to keep ink from leaking. Accordingly, back pressure within the ink-jet pen has to be regulated whenever ambient pressure drops. Also the pressure within the pen is subjected to what may be termed "operational effects". It is because the depletion of ink from the ink reservoir increases the reservoir back pressure. Without regulation of this back pressure increase, the ink-jet pen will fail soon because the back pressure is too high that the print head can not overcome it to eject ink drops.
Conventionally, the back pressure within the ink reservoir is controlled by a mechanism referred to as accumulators. In general, an accumulator includes an elastomeric bag capable of moving between a minimum volume position and a maximum volume position is response to changes in the back pressure within the ink reservoir. For example, as ambient pressure drops so that back pressure within the reservoir decreases simultaneously, the accumulator will move to increase the reservoir volume to thereby increase the back pressure to a level that prevent ink leakage. Another example is the depletion occurring during operation of the pen. In such a case, accumulators will move to decrease the reservoir volume to reduce the back pressure to a level within the operation range, thereby permitting the print head to continue ejecting ink.
However, although the accumulators such as elastomeric bags can adjust automatically the reservoir volume to keep the back pressure within the operation range, the extent to which elastomeric bags are capable of expanding is quite limited. Consequently, when ink gradually drops from the print head, the bag may reach its maximum extent and therefore incapable of any further adjustment of the reservoir volume. Hence, back pressure within the reservoir may increase such that ink droplets are prevented from coming out of the print head.
To resolve the aforementioned problems, some ink-jet pens employ a device called a "bubble generator". The bubble generator has an orifice through which ambient air can enter the reservoir. The dimension of the orifice is such that ink is trapped within the orifice to seal off the reservoir by capillary effect. When ambient air pressure is high enough to overcome the liquid seal, air can bubble into the ink-jet reservoir. Therefore, back pressure within the reservoir can decrease and capillary effect will take over and re-establish the liquid seal again to prevent entrance of more air bubbles.
In general, bubble generators of ink-jet pens must satisfy a few conditions. Firstly, the bubble generator must be able to control back pressure precisely. Secondly, The range of fluctuation of the back pressure within the reservoir must be as small as possible. In other words, as air bubbles enter the reservoir leading to a drop of back pressure, the bubble generator must be able to stop the entrance of bubbles soon enough so that a suitable back pressure remains inside. Thirdly, the bubble generator must have self-wetting capability. The liquid seal must be able to prevent the entrance of bubbles even when most of the ink within the reservoir is used up, or alternately when the ink-jet pen is tilted so much that the bubble generator is no longer immersed below the ink.
FIG. 1 is a cross-sectional diagram showing a conventional design of the bubble generator according to U.S. Pat. No. 5,526,030. The bubble generator installed within the reservoir 102 has an orifice 104 and a sphere 106. FIG. 2 is top view showing the surrounding structure of the bubble generator. As show in FIG. 2, the internal sidewalls of the orifice 104 contains equidistantly spaced protruding ribs 108 for centering the sphere 106. The circular gap 110 between the sphere 106 and the orifice 104 is location where ambient bubbles are produced.
Normally, a bubble generator such as above is able to meet the demands required for printing with an ink-jet pen. In general, the entrance of bubbles into the ink-jet pen 102 is determined by surface tension of the ink itself, static pressure of the ink column and the gap 110 between the sphere 106 and the orifice 104. Usually, the greater the surface tension of the ink or smaller the gap between the sphere and the orifice, the higher will be the back pressure required within the reservoir before air bubbles will start to enter. In addition, static pressure of the ink column within the reservoir can affect the value of back pressure required before air bubbles begin to enter the reservoir. Therefore, as ink gradually drops, static pressure of the ink column will decrease leading to the entrance of air bubbles at a smaller back pressure. In summary, major drawbacks of the aforementioned pressure control technique includes:
1. The value of back pressure within the ink-jet reservoir before bubble generator starts to function is related to surface tension of the ink used. Since various ink may have different surface tension, the minimum back pressure under which air bubbles can enter the reservoir may be different for each type of ink. Consequently, the gap between the sphere and the orifice must be designed for various ink.
2. The value of back pressure within the reservoir before bubble generator starts to function is also related to the static pressure generator by the column of ink. As ink within the reservoir drops gradually, static pressure acting on the bubble generator will drop making it easier for air bubbles to enter the reservoir. Often this will lead to a lowering of back pressure within the reservoir, and the adjustable range of the accumulator will be reduced.
3. The gap between the sphere and the orifice has to be precisely engineered to permit the entrance of air bubbles at the correct back pressure within the reservoir. This will increase difficulties in fabricating the reservoir of an ink-jet pen.
In light of the foregoing, there is a need to provide a better pressure control device within an ink-jet reservoir.